Method and solution for preparing SEM samples for low-K materials

ABSTRACT

A method and a solution for preparing SEM samples comprising low-K dielectric materials. The process begins by providing a SEM sample comprising low-K dielectric material and silicon oxide material. A solution is formed for preparing (staining and etching) the SEM sample by adding NH 4 F (s) to a solution comprising CH 3 COOH having a concentration of about 98% at a ratio of about 1 g NH 4 F (s):20 ml CH 3 COOH, then stirring until the NH 4 F (s) is thoroughly dissolved. Alternatively, the NH 4 F (s) can be added to a solution comprising HNO 3  having a concentration of about 70% and CH 3 COOH having a concentration of about 98%, with a volume ratio of about 15 ml HNO 3 :20 ml CH 3 COOH. The NH 4 F (s) is added at a ratio of about 1 g NH 4 F (s):35 ml CH 3 COOH and HNO 3 , and stirred until the NH 4 F (s) is thoroughly dissolved. The SEM sample is then etched in this solution for about 3 seconds, whereby the low-K dielectric material and silicon oxide material have similar etch rates with good selectivity to metals.

BACKGROUND OF INVENTION

[0001] 1) Field of the Invention

[0002] This invention relates generally to semiconductor fabrication andmore particularly to a method and chemical solution for preparing SEMsamples comprising low-K dielectric materials.

[0003] 2) Description of the Prior Art

[0004] For advanced generation semiconductors (0.18 microns and beyond),RC delay (resistance×capacitiance) begins to dominate overall devicedelay. Copper lines and low-K dielectric materials are attractive forreducing RC delay, and thereby enhancing performance. Chemical vapordeposited fluorinated oxide (FSG), hydrogen silsesquioxane (HSG), andmethylsilsesquioxane (MSQ) spin-on materials are particularly attractivelow-K dielectric materials for sub-quarter micron applications that aresensitive to RC delay, such as inter-level dielectric (ILD) layers.

[0005] However, due to different material characteristics, the etch ratefor spin-on low-K materials is typically faster than the etch rate fortraditional silicon oxide (SiO_(x)), making it difficult to prepare SEMsamples.

[0006] The importance of overcoming the various deficiencies noted aboveis evidenced by the extensive technological development directed to thesubject, as documented by the relevant patent and technical literature.The closest and apparently more relevant technical developments in thepatent literature can be gleaned by considering the following patents.

[0007] U.S. Pat. No. 4,733,074 (Kato et al.) shows a SEM sample surfacestructure having an insulating layer over the substrate.

[0008] U.S. Pat. No. 5,726,454 (Chun) shows a tripod for preparing a SEMsample.

[0009] U.S. Pat. No. 5,352,898 (Mehta) shows a method and apparatus forpreparing slurry specimens for SEM.

[0010] Also, further relevant technical developments can be gleaned byconsidering the following articles.

[0011] Peters, “Pursuing the Perfect Low-K Dielectric,” SemiconductorInternational, September 1998, pp. 64-74, discusses low-K dielectrics,including HSG.

[0012] Albrecht, “Material Issues with Thin Film Hydrogen SilsesquioxaneLow-K Dielectrics, J. Electrochem. Soc. Vol. 145, No. 11, November 1998,pp. 4019-4024, discusses chemical composition of HSG and its effect ondielectric constant and thermal stability.

SUMMARY OF THE INVENTION

[0013] It is an object of the present invention to provide a method andchemical solution for preparing SEM samples comprising low-K spin-ondielectric materials.

[0014] It is another object of the present invention to provide a methodand chemical solution for staining SEM samples which provides adecreased difference in etch rates of low-K spin-on dielectric materialsand convention silicon oxides. It is yet another object of the presentinvention to provide a method for preparing a chemical solution forstaining SEM samples which provides a decreased difference in etch ratesof low-K spin-on dielectric materials and convention silicon oxides.

[0015] To accomplish the above objectives, the present inventionprovides a method and a solution for preparing SEM samples comprisinglow-K dielectric materials. The process begins by providing a SEM samplecomprising low-K dielectric material and silicon oxide material. Asolution is formed for preparing (staining and etching) the SEM sampleby adding NH₄F (s) to a solution comprising CH₃COOH having aconcentration of about 90% at a ratio of about 1 g NH₄F (s):20 mlCH₃COOH, then stirring until the NH₄F (s) is thoroughly dissolved.Alternatively, the NH₄F (s) can be added to a solution comprising HNO₃having a concentration of about 70% and CH₃COOH having a concentrationof about 90%, with a volume ratio of about 15 ml HNO₃:20 ml CH₃COOH. TheNH₄F (s) is added at a ratio of about 1 g NH₄F (s):35 ml CH₃COOH andHNO₃, and stirred until the NH₄F (s) is thoroughly dissolved. The SEMsample is then etched in this solution for about 3 seconds.

[0016] The present invention provides considerable improvement over theprior art. Most importantly, SEM samples comprising low-K dielectricmaterials and silicon oxides can be prepared wherein the low-Kdielectric material and silicon oxide material have similar etch rateswith good selectivity to metals.

[0017] The present invention achieves these benefits in the context ofknown process technology. However, a further understanding of the natureand advantages of the present invention may be realized by reference tothe latter portions of the specification and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The features and advantages of a semiconductor device accordingto the present invention and further details of a process of fabricatingsuch a semiconductor device in accordance with the present inventionwill be more clearly understood from the following description taken inconjunction with the accompanying drawings in which like referencenumerals designate similar or corresponding elements, regions andportions and in which:

[0019] FIGS. 1 illustrates a SEM sample comprising a low-K spin-ondielectric material and a silicon oxide material having a first problemidentified by the inventor; wherein the low-K spin-on dielectricmaterial is completely removed.

[0020]FIG. 2 illustrates a SEM sample comprising a low-K spin-ondielectric material and a silicon oxide material having a second problemidentified by the inventor, wherein adjacent metal structures areetched.

[0021]FIG. 3 illustrates a SEM sample prepared according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention will be described in detail with referenceto the accompanying drawings. The present invention provides a methodand a solution for preparing SEM samples with low-K dielectric materials(e.g. HSG and MSG) and silicon oxide materials wherein the low-Kdielectric materials and silicon oxide materials have similar etch ratesand selectivity to metal.

[0023] Problem Identified by the Inventors—FIGS. 1 & 2

[0024]FIG. 1 illustrates a process determined by the inventor to have afirst problem, wherein a SEM sample (11) comprising a low-K spin-ondielectric material (not shown), a metal structure (13), aliner oxide(15), and a dielectric layer (e.g. ILD, IMD) (12) composed of siliconoxide is prepared (stained and etched) using a solution comprising 10 ml50% HF, 200 ml 98% CH₃COOH, and 200 ml 70% HNO₃. In FIG. 1 the SEMsample is a cross section of a gate (8) on a semiconductor structure(7). The metal structure (13) is one of a plurality of conductive plugswhich are separated by a low-K dielectric material (not shown). Thefirst problem identified by the inventor is that the slope solutionetches the low-K spin-on dielectric material too fast (i.e. too high aselectivity of the low-K spin-on dielectric material relative to theoxide liner (15) and dielectric layer (12)), removing excess amounts ofthe low-K spin-on dielectric material as shown in FIGS. 1A-1A.

[0025]FIG. 2 illustrates a process determined by the inventor to have asecond problem, wherein a SEM sample (11) comprising a low-K spin-ondielectric material (not shown), a dielectric layer (12) composed ofsilicon oxide, a liner oxide (15), and a metal structure (13) isprepared (stained and etched) using a 200:1 buffered oxide etch (BOE)solution. The second problem identified by the inventor is that the BOEsolution does not provide adequate etch selectivity of the dielectricsrelative to the metal structure, resulting in undesirable metal removalas shown in FIG. 2A. If the metal structure (13) does not remain thesame after the stain (i.e. the metal structure can not be etched by thestain process), then the X-SEM will not provide the desired information.

[0026] Preferred Embodiment of the Present Invention—FIG. 3

[0027] The preferred embodiment of the present invention begins byproviding a SEM sample (11), such as a semiconductor structure havingthereon a low-K spin-on dielectric material (14), a dielectric layer(12) preferably composed of silicon oxide, a liner oxide (15), and ametal structure (13). The low-K spin-on dielectric material preferablyhas a composition with silicon-hydrogen (Si—H) bonds, such as hydrogensilsesquioxane (HSQ) or methylsilsesquioxane (MSQ).

[0028] The SEM sample (11) is then prepared (stained and etched) using anovel solution formed by dissolving NH₄F (s) in a solution comprisingCH₃COOH at a concentration of between about 90% and 100%, preferablyabout 98%. The SEM sample (11) is preferably etched using this solutionfor a time of between about 1 second and 5 seconds, preferably about 3seconds. A key advantage of the present invention is that the etch ratefor low-K dielectric materials containing silicon-hydrogen bonds usingthe present invention is slower than the etch rate using traditionalsilicon oxide etching solutions, while the selectivity to metals ismaintained. The result is a SEM sample (11) as shown in FIG. 3A whereinthe similar etch rates for the dielectric layer (12) composed of siliconoxide, a liner oxide, and the low-K dielectric layer (14) provide arelatively planer cross-section, without metal loss due to poorselectivity to metal.

[0029] The solution for preparing SEM samples according to the presentinvention can be prepared by adding NH₄F (s) in a solution comprisingCH₃COOH having a concentration of between about 90% and 100%, preferablyabout 98%, at a ratio of between about 0.5 g NH₄F (s):20 ml CH₃COOH and2 g NH₄F (s):20 ml CH₃COOH and HNO₃, preferably about 1 g NH₄F (s):20 mlCH₃COOH. The solution is stirred until the NH₄F (s) is thoroughlydissolved.

[0030] Alternatively, the solution for preparing SEM samples accordingto the present invention can be prepared by adding HNO₃ having aconcentration of between about 50% and 96%, preferably 70% to CH₃COOHhaving a concentration of between about 90% and 100%, preferably 98%.The volume ratio is between about 10 ml HNO₃:30 ml CH₃COOH and 20 mlHNO₃:10 ml CH₃COOH, preferably 15 ml HNO₃:20 ml CH₃COOH. Then, NH₄F (s)is added to the solution comprising HNO₃ and CH₃COOH. The ratio of NH₄F(s) added to the solution comprising HNO₃ and CH₃COOH is between about0.5 g NH₄F (s):35 ml CH₃COOH and HNO₃ and 2 g NH₄F (s):35 ml CH₃COOH andHNO₃. The solution is stirred until the NH₄F (s) is thoroughlydissolved.

[0031] While the invention has been particularly shown and describedwith reference to the preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be made without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A method for forming a solution for preparing SEMsamples which provides an etch rate for low-K spin-on dielectricmaterials comparable to the etch rate for silicon oxides, comprising thesteps of: a. adding NH₄F (s) to a solution comprising CH₃COOH having aconcentration of between about 0.90% and 100% at a ratio of betweenabout 0.5 g NH₄F (s):20 ml CH₃COOH and 2 g NH₄F (s):20 ml CH₃COOH; andb. stirring said NH₄F (s) into said solution comprising CH₃COOH untilthe NH₄F (s) is disolved.
 2. The method of claim 1 wherein said solutioncomprising CH₃COOH further comprises HNO₃ and is formed by adding HNO₃having a concentration of between about 50% and 96% to CH₃COOH having aconcentration of between about 90% and 100% at a volume ratio of betweenabout 10 ml HNO₃:30 ml CH₃COOH and 20 ml HNO₃:10 ml CH₃COOH.
 3. Themethod of claim 2 wherein said NH₄F (s) is added to said solutioncomprising CH₃COOH and further comprising HNO₃ at a ratio of betweenabout 0.5 g NH₄F (s):35 ml CH₃COOH and HNO₃ and 2 g NH₄F (s):35 mlCH₃COOH and HNO₃.
 4. A method for preparing SEM samples comprising low-Kdielectric materials, comprising the steps of: a. providing a SEM samplecomprising low-K dielectric material and silicon oxide material; and b.etching said SEM sample in a solution comprising NH₄F (s) dissolved in asolution comprising HNO₃ having a concentration of between about 50% and96% and CH₃COOH having a concentration of between about 90% and 100% ata volume ratio of between about 10 ml HNO₃:30 ml CH₃COOH and 20 mlHNO₃:10 ml CH₃COOH; said NH₄F (s) being added at a ratio of betweenabout 0.5 g NH₄F (s):35 ml CH₃COOH and HNO₃ and 2 g NH₄F (s):35 mlCH₃COOH and HNO₃.
 5. The method of claim 4 wherein said SEM sample isetched in said solution for a time of between about 1 second and 5seconds.
 6. The method of claim 4 wherein said low-K spin-on dielectricmaterial comprises hydrogen silsesquioxane (HSG).
 7. The method of claim4 wherein said low-K spin-on dielectric material comprisesmethylsilsesquioxane (MSQ).
 8. The method of claim 5 wherein said low-Kspin-on dielectric material comprises hydrogen silsesquioxane (HSG). 9.The method of claim 5 wherein said low-K spin-on dielectric materialcomprises methylsilsesquioxane (MSQ).
 10. A method for preparing SEMsamples comprising low-K dielectric materials, comprising the steps of:a. providing a SEM sample comprising low-K dielectric material andsilicon oxide material; and b. etching said SEM sample in a solutioncomprising NH₄F (s) dissolved in a solution comprising CH₃COOH having aconcentration of between about 90% and 100% at a ratio of between about0.5 g NH₄F (s):20 ml CH₃COOH and 2 g NH₄F (s):20 ml CH₃COOH.
 11. Themethod of claim 10 wherein said SEM sample is etched in said solutionfor a time of between about 1 second and 5 seconds.
 12. The method ofclaim 10 wherein said low-K spin-on dielectric material compriseshydrogen silsesquioxane (HSG).
 13. The method of claim 10 wherein saidlow-K spin-on dielectric material comprises methylsilsesquioxane (MSQ).14. The method of claim 11 wherein said low-K spin-on dielectricmaterial comprises hydrogen silsesquioxane (HSG).
 15. The method ofclaim 11 wherein said low-K spin-on dielectric material comprisesmethylsilsesquioxane (MSQ).